This invention relates to electron or vacuum tubes in general and, more particularly, to a gas circulation means which can be employed in conjunction with a gas filled electron tube, such as a thyratron of ceramic construction.
The prior art is replete with a number of gas filled electron tubes which have been employed extensively as high power devices. Early design tubes, as one is well aware of, are fabricated from glass and have glass envelopes, for example, and other glass supporting structures. There is, however, a modern variety of high power vacuum tubes which are fabricated from ceramic materials. A typical example of such a gas filled electron tube is the thyratron. Essentially, a thyratron has a grid placed between the anode and cathode and is a hot cathode, gas filled, switch tube having such a grid.
Pulsed outputs of a thyratron may be controlled by means of a positive DC voltage which is applied to the thyratron grid. A voltage greater than critical must be applied to permit conduction but after conduction starts, reduction in grid voltage does not affect conduction. This action produces a high peak to average load current ratio. Hence, thyratrons are manufactured in most of the gas filled switch tube sizes with ratings up to tens of thousands of volts in regard to peak forward and inverse voltages together with extremely large ampere ratings.
As indicated above, many thyratrons employ glass envelopes and other glass supporting structures while certain higher power thyratrons employ typical ceramic structures. The manufacture and construction of both glass and ceramic thyratrons are well known in the art. It is immediately noted that a thyratron is one example of a gas filled tube to which the present invention applies in general.
In any event, a distinct advantage of a glass envelope thyratron construction over a comparable ceramic device of similar rating is the neutral (non-ionized) gas flow path which shunts the main discharge and is contained within a metal/mesh, internal axial structure. This annular cross-sectional volume, located coaxially between the outside of the discharge containment structure and the inner glass envelope wall, provides a source path for neutral molecular hydrogen, from the gas reservoir located at the base of the tube, to the anode-grid region, where electronics are pumped. Gas is collisionally ionized in this tube region and heat is generated during the commutation and conduction phases of the thyratron operation. Maximum thyratron operating frequency is limited by the recombination (recovery) time. Recombination, a three body boundary surface reaction, is a strong function of long lived atomic, metastable and rotational energy gas species. It is expedited by the available "cold", neutral gas molecules flowing into the gap region through the electron mean-free-path (mfp) diameter holes of the mesh structure, into the discharge bulk volume of the gas tube.
Additionally, the maximum conduction current rating is a function of the neutral gas available for ionization (to prevent gas starvation) and thus switching capability is favorably enhanced by the above-described mechanism.
In any event, there is no "outside the discharge" path in a conventional ceramic envelope device. In such a device all gas flow must pass through the constrictions in the grid apertures and the baffle holes.
It is, therefore, an object of the present invention to create a parallel neutral, low temperature, molecular gas circulation path while concurrently avoiding long path discharge (PASCHEN breakdown) in a ceramic envelope device.
It is a further object to provide an "outside the discharge" path for a ceramic envelope device which will enable such a device to have the beneficial gas circulation characteristics of a glass envelope device while capable of more rugged operation, together with a higher power ceramic metal structure.